1. Field of the Invention
The present invention relates to rate responsive cardiac pacemakers and more particularly to the use of a DC accelerometer for detection of patient posture and activity level, particularly to provide appropriate pacing rates during stair climbing and descending.
2. Description of the Prior Art
Rate responsive pacing has been widely adopted for adjusting pacing rate to the physiologic needs of the patient in relatively recent years. Early single chamber cardiac pacemakers provided a fixed rate stimulation pulse generator that could be reset, on demand, by sensed atrial or ventricular contractions recurring at a rate above the fixed rate. Later, dual chamber demand pacemakers became available for implantation in patients having an intact atrial sinus rate but no AV conduction, so that ventricular pacing could be synchronized with the atrial sinus rate, and backup fixed rate ventricular pacing could be provided on failure to sense atrial depolarizations. In addition, rate programmable pacemakers became available wherein the base pacing rate could be selected by a physician to provide a compromise fixed rate that did not interfere with patient rest and provided adequate cardiac output at moderate levels of exercise.
Such fixed rate pacing, particularly for patients not having an adequate atrial sinus rate to allow synchronous pacing, left most patients without the ability to exercise, lift objects or even walk up stairs without suffering loss of breath due to insufficient cardiac output. However, the introduction of the Medtronic.RTM. Activitrax.RTM. pacemaker provided patients with the a pulse generator having a rate responsive capability dependent on the level of patient activity. A piezoelectric crystal bonded to the interior of the implantable pulse generator can or case is employed in that pacemaker and successor models to provide a pulse output signal related to the pressure wave generated by a patient's footfall and conducted through the body to the crystal. Thus, low frequency activity signals recurring at the patient's rate of walking or running could be sensed and processed to derive a pacing rate appropriate to the level of activity. The activity sensor and its operation is described in commonly assigned U.S. Pat. No. 4,428,378 to Anderson.
Since the introduction of the Activitrax.RTM. pacemaker, a great many rate responsive pacemakers employing a wide variety of activity sensors and other physiologic sensors have been proposed and marketed. A comprehensive listing of such rate responsive pacemakers, sensors and sensed physiologic parameters is set forth in commonly assigned U.S. Pat. No. 5,226,413 to Bennett et al., incorporated herein by reference. However, the activity sensor of the type employed in the Activitrax.RTM. pacemaker continues to be used in successor single and dual chamber, rate responsive pacemaker models and remains the most widely used physiologic sensor.
As mentioned above, this piezoelectric crystal sensor is responsive to pressure waves generated by patient footfalls striking the exterior of the pulse generator case. Activity sensor configurations employing integrated circuit, AC accelerometers on an IC chip inside the pacemaker are also being employed in the EXCEL"VR pacemaker sold by Cardiac Pacemakers, Inc., and in similar rate responsive pacemakers sold by other manufacturers. The AC accelerometer is formed of a silicon beam mass suspended on the IC that swings or moves in response to shock waves caused by body motion and provides an output signal having a magnitude dependent on the rate of movement.
The relative virtues and weaknesses of piezoelectric crystal and AC accelerometer activity sensors and associated pacemakers are reported widely, e.g. in the article "Activity-Based Pacing: Comparison of a Device Using an Accelerometer Versus a Piezoelectric Crystal", by Bacharach et al. (PACE, Vol 15, pp.188-196, February 1992). As indicated in that article, the pacing rate responses of these pacemakers strapped on patients with normal hearts who were subjected to various stress tests were measured and compared to each other and to the patients' average actual heart rates. The tests conducted included stair ascending or climbing and descending tests, and conclusions were drawn to the effect that the AC accelerometer performed superiorly to the piezoelectric sensor in certain respects. Higher cardiac output is required in ascending a flight of stairs than in walking at the same rate or in descending the flight of stairs at the same rate as indicated by the patients' heart rates. The reported AC accelerometer induced pacing rate during stair climbing more closely matched the required cardiac output as indicated by the test subjects' average heart rates. During stair descending, the AC accelerometer induced pacing rate did not appreciably fall and exceeded the patients' actual heart rate. The reported piezoelectric sensor induced pacing rate during stair climbing fell below the required cardiac output as indicated by the test subjects' average heart rates. During stair descending, the piezoelectric crystal induced pacing rate increased from the rate achieved during ascending and also exceeded the patients' heart rate.
As a result, while the authors suggest that the AC accelerometer is superior in certain respects to the piezoelectric crystal sensor, the test data also indicates that the AC accelerometers do not adequately distinguish between stair ascending and descending or walking at the same rate on a flat surface to set an appropriate pacing rate. Neither the AC accelerometer nor the piezoelectric sensor can inherently distinguish these patient activities. If an appropriate rate for an individual patient is set for stair climbing, for example, that rate may only be triggered by the frequency of recurrence of the patient footfalls and consequently may be too high a rate for either stair descending or level walking at the same speed.
Like the piezoelectric crystal sensor, there is no signal output from the AC accelerometer in the absence of body motion and related to body position or attitude. In other words, when a patient is at rest, neither activity sensor provides any indication as to whether the patient is upright and awake and resting or lying down and presumably sleeping or resting. Other sensors for sensing physiologic parameters induced by high levels of exercise have been proposed to detect the physiologic changes accompanying exercise, rest and sleep to trigger appropriate rates. To lower the pacing rate during sleep, the inclusion of a real time clock to establish a Circadian rhythm pacing rate has also been proposed. None of these proposed sensors or systems are capable of determining a patient's position or posture.
A mechanical sensor has been proposed in the article "A New Mechanical Sensor for Detecting Body Activity and Posture, Suitable for Rate Responsive Pacing" by Alt et al. (PACE, Vol.11, pp. 1875-81, November, 1988, Part II) and in U.S. Pat. No. 4,846,195 that involves use of a multi-contact, tilt switch. This switch employs a mercury ball within a container that is proposed to be fixed in the pulse generator case, so that if the pulse generator is implanted at a certain orientation, and stays in that orientation, certain contacts are closed by the mercury ball when the patient is upright and others are closed or none are closed when the patient is prostrate, i.e., either prone or supine. During movement of the body, the mercury ball is expected to jiggle randomly and the number of contacts made per unit of time may be used as a measure of the level of activity. Similar sensors have been proposed in U.S. Pat. Nos. 4,869,251, 5,010,893, 5,031,618 and 5,233,984.
The use of elemental mercury is generally not favored and would increase environmental problems related to disposal of the pulse generators after use. Long term contact contamination and bridging issues would also arise, particularly given the extremely small size of the switch for confinement within modern pulse generator cases.
Presumably, the multi-contact tilt switch sensor would also not necessarily be able to distinguish between stair climbing and descending at the same stepping rate. Given the necessary small size of the tilt switch, it would be difficult to accurately position the pacemaker pulse generator so that consistent, reproducible signal outputs from the sets of contacts bridged while stooped forward or rearward would be achieved in a given patient over time. Moreover, the limited number of contacts reduce the possibility that such discrimination could be achieved. To date, no implants of pacemaker pulse generators using such a tilt switch have been reported.
More recently, the use of a solid state position sensor in the form of a DC accelerometer is proposed in U.S. Pat. No. 5,354,317. The DC accelerometer disclosed in the '317 patent is fabricated in hybrid semiconductor IC form as a polycrystalline silicon, square plate, suspended at its four corners above a well in a single silicon crystal substrate, and associated low pass filter circuits are formed on the same substrate. The suspended plate structure moves between stationary positions with respect to the well on the suspension arms in response to earth gravity, depending on its orientation to the gravitational field. The plate also vibrates on the suspension arms similar to the AC accelerometer in response to acceleration movements of the patient's body.
The single DC accelerometer of the '317 patent is oriented to be sensitive to the anterior-posterior axis of the patient so that the upright, supine and prone body positions can be discriminated, and separate base pacing rates can be set. Rate changes from the base pacing rates dependent on the exercise level of the patient in each position are suggested. When changes in patient position are detected in the absence of physical exercise, the base pacing rate change is smoothed between the old and new rate to avoid a sudden step change.
The signal processing of the output signal from the single DC accelerometer of the '317 patent includes signal level calibration for each individual patient to account for differences in the angle of orientation of the DC accelerometer plate resulting from the implantation angle of the pulse generator case in the patient's body. However, this calibration is not suggested in order to distinguish body positions having a more or less common angular relation of the movable plate to the gravitational field.
In addition, the '317 patent does not appear to suggest any discrimination of stair climbing that would alleviate the problems identified above resulting in the same or a higher pacing rate being developed during stair descending than during stair climbing.
Despite the weaknesses reported with respect to the piezoelectric sensors and solid state accelerometers, they remain favored over the other physiologic sensors that have been proposed or are in clinical use due to their relative simplicity, reliability, predictability, size, and low cost.
Problems to be Solved by the Invention
In view of the demonstrated advantages of the piezoelectric and AC accelerometer type activity sensors, it would be desirable to employ solid state sensors responsive to patient activity in a similar manner that would also distinguish stair or steep incline climbing from other activities in order to provide an appropriate rate response to provide adequate cardiac output.